Dual or multi-energy spectral computed tomography (CT) systems can reveal the densities of different materials in an object and generate images acquired at multiple monochromatic x-ray energy levels. In the absence of object scatter, a system derives the behavior at a different energy based on a signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region of medical CT, two physical processes dominate the x-ray attenuation: Compton scattering and the photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials.
In some cases, the presence of metal (e.g., in the form of metal implants, dental fillings, and so on) may interfere with the x-ray attenuation, thereby causing metal artifacts in reconstructed images. For single energy acquisition, there are known metal artifact reduction algorithms which effectively reduce the presence of metal artifacts in the reconstructed image.
However, a simple application of known metal artifact reduction algorithms to dual energy CT leads to additional image artifacts. For example, a metal artifact reduction algorithm known to work well for single energy acquisition may be applied independently to both the high and the low channels in dual energy CT. As a result, different amounts of metal correction may occur in each channel. Material decomposition utilizes data from both channels, and new artifacts arise due to the inconsistency of metal correction between the channels.